1. Field of the Invention
This invention relates to an apparatus for indicating the loading of an electric AC motor, and more particularly to such an apparatus for asynchronous motors.
There are a great many domains where a simple apparatus for indicating the loading of AC motors and variations in said loading would be extremely valuable and could facilitate and improve the control of various processes or operations dependent upon the working conditions of the motor. Mention may be made for instance of supervision to avoid idling or overloading, indication of the completion of a work cycle such as in drilling, thread cutting and sawing operations, possibly combined with the release of another work cycle, and control of the power of a load in dependence of the loading of an electric AC motor.
2. Description of the Prior Art
For indication of the loading of an AC motor use has earlier been made of devices indicating the motor current or the motor power, but these devices suffer from disadvantages among other things as a consequence of poor sensibility to load variations within parts of the load range. An indication of the loading of an AC motor can also be realized by means of a device for generating a signal the magnitude of which is a function of the phase difference between the motor current and the motor voltage, since said phase difference is substantially linearly related to the loading. One disadvantage from which such a phase difference detecting device suffers, is in some cases its sensitivity to variations in the supply voltage of the motor.